1. Field of the Invention
The present invention relates to an electrodialysis apparatus for purifying water and also embraces a method of electrodialysis.
2. Description of the Related Art
As will be well known to those skilled in the art, electrodialysis comprises passing a desalting stream of water to be purified between an anode and a cathode. Perm-selective membranes are positioned between the desalting stream and the electrodes, such that cations migrating towards the cathode pass through a cation selective membrane, whilst anions migrating towards the anode pass through an anion perm-selective membrane. As electrodialysis proceeds, the water to be purified is progressively depleted of ions. In some instances, an ion exchange resin bed is placed in the desalting stream, and this has the advantage of maintaining the electrical conductivity of the water to be purified as deionisation proceeds. Electrodialysis in which an ion exchange resin bed is used in the water to be purified is known as electrodeionisation.
U.S. Pat. No. 3,645,884 (Gilliland) discloses a five compartment electrodeionisation cell comprising outer anode and cathode compartments and a central concentrating compartment. A desalting compartment is positioned between each electrode compartment and the central concentrating compartment. The desalting compartment nearest the anode is separated from the anode chamber and from the central concentrating compartment by two, respective cation selective membranes. Similarly, the desalting compartment nearest the cathode is separated from the cathode chamber and from the concentrating compartment by two, respective anion selective membranes.
In operation, water to be purified is passed through the two desalting compartments in succession, and a voltage is applied across the anode and cathode, such that cations are caused to migrate from the water to be purified in the desalting compartment nearest the anode into the central concentrating compartment, and anions are caused to migrate from the water to be purified in the desalting compartment nearest the cathode into the concentrating compartment. Each of the desalting compartments comprises a porous bed of ion exchange material.
According to U.S. Pat. No. 3,645,884, the anode compartment is filled with a weakly acidic solution, whilst the cathode department is filled with a weakly alkaline solution. Thus, in operation, hydronium ions from the anode compartment are caused or allowed to pass into the adjacent desalting compartment to assist in driving out the cation impurities in the water to be purified. Analogously, hydroxide ions in the cathode compartment are caused or allowed in use to pass into the juxtaposed desalting compartment to assist in driving out the anion impurities from the desalting stream into the central concentrating chamber.
U.S. Pat. No. 3,645,884 discloses that when a 0.085N sodium chloride solution is deionised for a period of two hours by impressing a current of 12 amperes at 10 to 20 volts across the ion exchange beds, a final product having a sodium chloride concentration of 0.0085N is achieved, indicating a 90% removal of ions from the water to be purified. The final product water has a conductivity of 1025 xcexcS/cm at 25xc2x0 C.
It is an object of the present invention to provide an improved electrodialysis apparatus.
In particular, it is an object of the present invention to provide an electrodialysis apparatus which is capable of producing highly-purified water, that is water having a conductivity of less than 0.20 xcexcS/cm, i.e. a resistivity of at least 5 Mxcexa9-cm and, under suitable operating and feed conditions, of at least 15 Mxcexa9-cm i.e. a conductivity of less than 0.067 xcexcS/cm at 25xc2x0 C.
According to one aspect of the present invention therefore, there is provided an electrodialysis apparatus for purifying water, which apparatus comprises means defining an anode chamber, means defining a cathode chamber, an anode means disposed within the anode chamber, a cathode means disposed within said cathode chamber, means defining a desalting stream flow path between said anode and said cathode means, means defining a concentrating stream flow path adjacent said desalting stream flow path, means for causing or allowing water to be purified to flow within the desalting stream flow path and means for causing or allowing a fluid adapted to receive ionic impurities from the water to be purified to flow within the concentrating stream flow path; wherein said desalting stream flow path comprises a first portion juxtaposed the anode means and a second portion juxtaposed said cathode means, and said means defining the desalting stream flow path comprises two first partition means that separate the first portion of the desalting stream flow path from the anode chamber and the concentrating stream flow path respectively, which first partition means are selectively permeable to cations and are spaced apart on an axis between said anode means and cathode means, and two second partition means that separate the second portion of the desalting stream flow path from the cathode chamber and the concentrating stream flow path respectively, which second partition means are selectively permeable to anions and are spaced apart on said axis; characterised in that each of said anode and cathode chambers contains a porous bed of ion exchange material.
In some embodiments, the cathode chamber may contain anion exchange resin. Said anode chamber may contain cation exchange resin.